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Zhao et al. Soft Sci 2024;4:18 https://dx.doi.org/10.20517/ss.2024.04 Page 13 of 32

[62]
consumption in practice . All these developments contribute to the field of personalized health monitoring
using sweat-based metabolite analysis.

Ions and sweat volume detection
Sweat can also provide significant perspectives into an individual’s health condition by reflecting the body’s
amount of salt and water metabolism. For instance, dehydration, cramping in the muscles, and
hyponatremia can result from excessive sodium and potassium loss. Emaminejad et al. proposed a wearable
electrochemical sensor [Figure 7A] as a wristband. It safely extracts sweat using improved iontophoresis,
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+
allowing thorough analysis of specific elements such as Na and Cl . The system wirelessly controls sweat
production and provides instant on-site measurements. Specialized electrochemical methods and ion-
selective membranes enhance electrode stability and selectivity. This advanced system enables easy access to
abundant sweat samples for widespread health monitoring . Parrilla et al. reported a wearable
[61]
+
potentiometric ion sensor (WPIS) to measure ions in sweat. These sensors track pH, Cl , K , and Na using a
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flexible sampling cell. The electronic part is screen-printed on a flexible substrate, and electrodes are
enhanced utilizing multi-walled carbon nanotubes (MWCNTs) and a selective membrane. The sampling
cell ensures accurate sweat capture without contamination or evaporation issues [Figure 7B] .
[107]
As for the sweat volume monitoring, a flexible, adhesive patch with sticker-like properties is attained by
combining multiple layers of microfluidics, reservoirs for dyes and bioassays, a top layer featuring color
reference stripes, and a skin adhesive layer underneath, as illustrated in Figure 7C. A specified sweat
collecting location can yield around 130 μL of sweat for Microchannel 1. Perspiration and an orange dye
combine to create a highly visible propagation down the channel, making it possible to quickly analyze and
estimate the volume of perspiration. The low bending stiffness and thin shape of the device allow for
[47]
mechanical deformations, improving wearability during strenuous physical activity . Then, Wang et al.
introduced an innovative wearable sensor for calibration-free assessment of regional sweat rate and total
electrolyte level. By employing a compact vertical channel, integrated conductance electrodes, and an
absorbent layer, the microfluidic-based sweat sensor device achieves controlled sweat behavior and
sequential detection. The sweat is collected in small volumes, contacts the electrodes, and evaporates
quickly. Real-time recordings generate a square-wave conductance profile, providing information on sweat
rate and electrolyte concentration. The sensor can measure sweat rate within the range of 0.5-
-1
-2
[110]
20 μL·min ·cm and electrolyte levels between 1-200 mM, with minimized flow resistance [Figure 7D] . A
tape-free device is introduced, featuring a 3D-printed sweat collector with a concave surface securely
strapped onto the skin. Optimized for efficient sweat capture and conformal contact, the collector interfaces
with embedded electrodes and a fluidic microchannel for continuous sweat rate monitoring. It accurately
measures long-term exercise-induced sweat rates from various body locations, offering insights into the
correlation between sweat profiles and overall fluid loss. With easy installation and reusability, it can be
[111]
seamlessly integrated with a watch band [Figure 7E] . Preventing damage to electronic components
caused by sweat is of utmost importance for effectively utilizing sweat sensors. Recently, extensive research
has been conducted to address this issue. One promising approach involves implementing sealing processes
that effectively block any liquid from entering the internal electronic components [61,79] . Additionally,
integrating intelligently designed microfluidic channels or permeable materials aids in efficiently directing
sweat toward the sensor or detection unit, thereby significantly reducing the likelihood of sweat reaching
and potentially harming the electronic components of the wristband [112,113] .

Drugs detection
Sweat analysis has also become a vital tool for drug analysis, finding applications in clinical therapies, drug
misuse testing, doping control, and digital health monitoring [26,114] . The wearable electrochemical aptasensor
[Figure 7F] detects drugs in sweat with good precision and sensitivity. It utilizes an array of single and dual